1. Field of the Invention
The present invention relates to a process for producing the cutting dies normally used in rotary and reciprocating die cutting machines. The dies are suitable for cutting a wide variety of shapes on paper, plastics, foils, diapers, etc.
2. Prior Art
It is usual to cut shapes from sheet material such as paper, plastics, and foils, using a cutting die which comprises a ridge or ridges of sharpened metal projecting from a base, and which cuts the material by pressing this against a plain anvil. Commonly, the dies are rotary, i.e. cylindrical, and are positioned in a die station equipped with an anvil roll, the material being cut when passing between the rotary die and the anvil roll while these rolls rotate in opposite directions. The ends of the die have slightly enlarged cylindrical surfaces termed "bearers" which contact suitable support surfaces to properly position the die. The die station also includes drives, support rolls, wiper assemblies for lubricant, and some means, usually hydraulic, to apply the proper cutting load between the rotary die and the anvil roll, and to adjust the height so that the machine can cut to a specific depth.
In some cases it is required that the dies be accurately enough made to cut only one or two outer layers of a paper laminate, leaving a backing layer uncut.
Cutting dies are usually made by machining metal away from a metal base to leave ridges which are then heat treated and sharpened. The machining operation is a major step and a major expense. For a rotary die, bar stock with a diameter slightly bigger than the biggest required diameter of the die is turned on a lathe. Required end and bearer diameters are machined, leaving the central area for the machining of the cutting pattern. This cutting pattern is first rough machined and then finished using a CNC (computer numerical control) milling machine. The entire die is made oversize to allow for the distortion which occurs during heat treatment. Depending on the complexity of the pattern and the height of the required cutting edges, the milling operation may take from a few hours to days or even weeks. For sharp corners, small diameter cutters are required, which further adds to the manufacturing time.
The machining operations are performed on the tool steel bar stock from which the die is made, when the material is in the relatively soft condition as initially received; the steel then requires heat treating to bring it to the required hardness of about RC 54-60. This usually means that the dies have to be sent out to a commercial heat treater. Although the heat treatment, namely hardening and tempering, can be done in a few hours, the return time from the heat treaters may be several days. Heat treatment is also problematical in that improper treatment may lead to cracking which destroys the die. Distortion also commonly occurs; this may need to be corrected or may mean that the die has to be scrapped.
After heat treatment, the die is ground to the finish dimensions on a cylindrical grinder. If the distortion caused by heat treatment is bad, centers have to be re-machined for proper grinding.
The final major operation is the sharpening process. The sharpened edges must be slightly below the bearer diameter. Edges are hand sharpened, by a scraping process, to give a sharpened edge at the specified diameter. The cross-section of the sharpened edge depends on the final application, and ranges from an inverted "V" with either acute or obtuse angle to a rounded top on the "V".
It will be clear that this standard machining operation is time consuming and expensive in relation to the amount of material which has to be removed, and that the need for heat treatment is also a disadvantage. The machining operation also imposes some limitations on shapes that can be cut, i.e. acute inside angles and sharp inside bends are not possible with conventional machining methods and may require special electro-machining.
The present invention provides a process which entirely or largely avoids the need for removing metal by machining; instead the invention provides for the addition of metal to a base. The need for conventional heat treatment is also avoided.